Avoid using vtkExtentTranslator from the pipeline.

vtkStructuredGridVolumeRepresentation no longer users the vtkExtentTranslator
provided by the pipeline to determine the piece-extent mapping. Instead it
explicitly determines that using parallel communication. Although this is not
essential currently, with forthcoming VTK changes, this will become essential
i.e. not relying on the extent translator from the pipeline.

Change-Id: I61ec582b7d7dc291f0da779489153b62411c4c26
Merge topic 'fix_structured_volume_rendering' into next

206986c Avoid using vtkExtentTranslator from the pipeline.